The present invention relates to a hollow body of revolution based on a thermoplastic organic material, in which continuous glass filaments, helically wound around its axis, are embedded.
Although the invention is not limited to such an application, it will be more particularly described with reference to the manufacture of pipes of all types, especially those intended for conveying pressurized fluids.
Another advantageous application is the manufacture of tanks intended to contain fluids, especially pressurized fluids.
Plastics have already been widely used for this type of manufacture, but, in order to withstand the high pressures encountered, the wall of the pipes manufactured must necessarily have a very great thickness. This very great thickness gives the pipes a considerable weight.
In order to reduce the weight of such pipes, it has already been proposed for them to be reinforced by means of reinforcing fibres arranged at their surface. Such a solution has been envisaged, for instance, by Patent GB-A-2,077,880 which describes a composite pipe consisting of an extruded former made of PVC or of polypropylene immediately covered, in the axial direction, with continuous glass yarns impregnated with a thermoplastic resin, helically wound around which are continuous glass yarns impregnated with a thermosetting resin, the latter yarns themselves being covered with a layer of thermoplastic resin.
Apart from the fact that the continuous manufacture of these pipes is expensive and very complex, the life of these pipes is very short. This is because the various aforementioned materials of which they are composed rapidly delaminate from each other, thereby rapidly reducing their burst strength.
This is why Patent EP-A-0,697,553 has proposed another type of composite pipe consisting of an extruded plastic former within which short reinforcing fibres are dispersed, parallel to its axis, and around which continuous reinforcing fibres are helically wound.
It turns out that the short fibres very rapidly become detached from the extruded former, thereby creating in the latter fracture initiators which propagate rapidly. Consequently, the burst strength of the pipe rapidly decreases. To guarantee a minimum long-term burst strength, especially one that meets the standards in force, it is then necessary to compensate for rapid propagation of fractures by again increasing the thickness of the extruded former, which results in a non-negligible increase in the weight of the pipe.
Furthermore, even if a minimum long-term burst strength is guaranteed, short fibres, once loosened, appear at the inner surface of the former. When the fluid intended to be conveyed by the pipe is water, the food-related criteria that the pipe must meet are no longer satisfied since the visible short fibres run the risk of contaminating the said fluid.
The object of the present invention is therefore to alleviate the aforementioned drawbacks and especially to propose a pipe of the aforementioned type which is lightweight and has an excellent long-term burst strength.
To do this, the subject of the invention is a hollow body of revolution intended especially for containing a pressurized fluid, the wall of the body comprising in its thickness:
a first region, the internal periphery of which is intended to be in contact with the fluid and at least the external periphery of which is made of a single thermoplastic organic material A, and
a second region made of a thermoplastic material identical to the material A and of continuous glass yarns which are embedded in the said thermoplastic material and are wound helically around the longitudinal axis of the hollow body of revolution,
characterized in that
the second region comprises in its thickness a first part arranged so as to be continuous with the first region of the body and made of the thermoplastic material A and the glass yarns, and a second part forming the external periphery of the body and made only of the thermoplastic material A, and
the wall of the said body has a volume void content Vv of less than 0.5%, preferably less than 0.2%.
It is specified that, within the context of the invention, the volume void content according to the invention may be measured in a known manner using a micrographic method of the image-analysis type. It is therefore appropriate to take several local measurements on the hollow body and to carry out a statistical analysis thereof.
It is also possible to calculate the volume void content differently, in a generalized way using the following formula:
Vv=(dthxe2x88x92dr)/dth
in which dth and dr represent the theoretical density and the actual density of the body of revolution, respectively. The theoretical density is calculated from the density of the glass and from the various densities of the thermoplastic organic material A weighted by their relative percentages. As regards the actual density, this is calculated by taking the ratio of the actually measured mass of the body of revolution to the actual volume of the body.
The solution completely solves the problem posed. To achieve this, the inventors have firstly been able to analyse the key functions that a composite pipe must fulfil and to demonstrate the shortcomings of the composite pipes according to the prior art, such as those mentioned in the preamble.
According to this analysis, the reinforcing fibres must in principle withstand all the hoop and axial stresses due to the pressure exerted by the fluid flowing inside the pipe and the internal region made of thermoplastic organic material must be impervious to and chemically compatible with this same fluid. A simple hooping of continuous reinforcing fibres over a former made of thermoplastic organic material should therefore fulfil these functions.
Now, the inventors have discovered that, in order to guarantee the durability of a composite pipe of this type over time, it is absolutely necessary for there to be both effective protection of the fibres and perfect adhesion between the various elements of which it is composed.
They have thus been able to devise a body, as claimed, with a sufficiently intimate union between the various elements of which it is composed that a lifetime much longer than those encountered hitherto can be guaranteed.
Furthermore, the invention makes it possible to obtain pipes which are much easier to transport and handle.
According to one advantageous characteristic of the invention, the thermoplastic organic material A may be a polyolefin, of the polyethylene PE or polypropylene PP type, or else polyvinyl chloride (PVC). These materials have the advantages, among others, of being chemically inert, of being able to withstand very low temperatures and of having a low manufacturing cost.
The density d of the thermoplastic organic material A may vary through the thickness of the wall and may, especially, be between 0.915 and 0.960 g/cm3.
According to an advantageous variant, the continuous glass yarns embedded in the thermoplastic organic material make an angle of between 50 and 55xc2x0 with the axis of the said body. Such an arrangement allows the axial and hoop resistance of the body to the pressure exerted by a fluid flowing through or contained within it to be further increased, for the same amount of continuous glass yarns used.
According to another advantageous variant, the continuous glass yarns embedded in the thermoplastic organic material make an angle close to 90xc2x0 with the axis of the body and other continuous glass yarns are embedded in the thermoplastic organic material, these being arranged longitudinally along the axis of the body.
The choice of one of these variants, of their combination or of another variant giving preference to a different angle of winding of the continuous glass yarns, as well as the choice of the respective amounts of the glass yarns in the direction in which they are placed, will be made according to the specific constraints associated with each application, such as the resistance to pressure, resistance to ovalization, flexural strength, tensile strength, etc.
Preferably, the continuous glass yarns are distributed uniformly in the first part of the second region of the wall of the hollow body. Such a distribution of the yarns in the thermoplastic organic material very favourably enhances the mechanical properties of the body and guarantees them in the long term.
The invention also relates to a composite pipe consisting of the body of revolution defined above, coated with an external finishing and protective layer made of thermoplastic organic material, preferably one identical to the material A.
The finishing layer according to the invention allows the pipe to be reliably protected against external attack likely to occur during storage, transportation, site operations and use.
The body or the pipe according to the invention are particularly suitable for containing and/or conveying pressurized fluids.
The invention also relates to a process for manufacturing a hollow body of revolution, the wall of which is based on a thermoplastic organic material A in which continuous glass yarns are embedded, these being wound helically around the axis of the body. According to this process, the following steps are carried out in line:
a) a tape in the heated state is wound helically around a rotating tube, at least the external face of the wall of which is based on the thermoplastic material A, the tape being made of the same thermoplastic material A and of continuous glass filaments embedded in this material;
b) part of the outer peripheral surface of the tube coated with the tape is heated in a zone located immediately downstream of the zone where the tape comes into contact with the tube, to a temperature above the melting point of the material A;
c) local pressure is applied to that part of the outer peripheral surface of the tube coated with the tape in a zone located immediately downstream of the heating zone of step b).
Advantageously, cooling is applied at the same time as the local pressure.
This process thus makes it possible to obtain a very good distribution of the glass filaments in the thermoplastic material by the use of a composite tape rather than a composite yarn as used in Patent Application EP 569,928.
Moreover, unlike the process of the invention which involves a heating step and a compression/cooling step downstream of the point where the tape is laid, the process of Application EP 569,928 reveals the need to simultaneously compress and heat the composite yarn on the tube when brining the yarn into contact with the said tube in order to make it easier for the turns of yarn to be bonded together and to expel the air between the filaments of the yarn, whereas, in the process of the invention, the heating is downstream, allowing the thermoplastic material of the tube and that of the tape, once the latter has been put into place, to fuse together, and the pressure, also applied downstream, makes it possible to remove the air between the layers of the tape but not at all between the glass filaments which are already contiguous, one against another, with no air being present because of the very nature of the product covering the tube, that is the tape.
Furthermore, the process of the invention is particularly advantageous to apply to a thermoplastic material of the polyolefin type, since, although this material has a high viscosity, and is therefore difficult to process, especially in combination with fibres as is pointed out in Application EP 569,928, the process succeeds perfectly in fusing the thermoplastic material of the tube with that of the composite material of the tape covering it. It is all the more important to stress this arrangement since polyolefin-type materials are advantageously of low cost and compatible from the food standpoint.
Preferably, the tape arrives for step a) in the heated state, having undergone, in a zone located near the tube, an operation of surface heating to a temperature above the softening temperature of the material A but below its degradation temperature.
Also preferably, prior to step a), the following steps are also carried out in line:
continuous comingled yarns, consisting of intimately mixed glass filaments and filaments of material A in the form of at least one sheet of parallel yarns, are led in and assembled;
the said sheet is introduced into a zone where it is heated to a temperature between the melting point and the degradation temperature of the material A;
the heated sheet is made to pass through an impregnation device so as to obtain a densified and laminated tape of flatter shape than the tape as in step a);
the laminated tape is introduced into a zone where it is heated to a temperature between the melting point and the degradation temperature of the material A so as to obtain the heated tape as in step a).
According to this variant of the process, it is possible to use windings of comingled yarns obtained in a direct process, such as that described in Patents EP 0,367,661, WO 98/01751 and EP 0,599,695, which yarns have the advantage of having an excellent comingling index, especially by virtue of the stability of the process. Within the context of the invention, the expression xe2x80x9cexcellent comingling indexxe2x80x9d should be understood to mean a comingling index whose mean value is less than 12. It should be pointed out that the mean value of the comingling index is calculated in the following manner:
a number of cross sections are made along a given length of comingled yarn;
a grid is placed over each of these sections;
the surface distribution of the glass filaments and the filaments of thermoplastic organic material is measured for each grid cell using a micrographic method of the image-analysis type;
for each section, the standard deviation of the surface distributions of each of the grid cells is calculated, this being the comingling index of the section in question;
the mean value of the comingling index for all the sections is calculated.
This excellent comingling index results in an excellent distribution of the glass yarns in the thermoplastic material parallel to the axis of the body with the concomitant advantages such as those mentioned above.
Finally, the invention relates to an apparatus for implementing the process defined hereinabove. This apparatus is notable in that it comprises:
means for at least surface heating a tape consisting of continuous glass yarns embedded in a thermoplastic organic material A, in a zone located near a tube, at least the external face of the wall of which is based on the same material A, rotating about its axis;
means for helically winding the heated tape around the rotating tube;
means for heating part of the outer peripheral surface of the tube coated with the tape, in a zone located immediately downstream of the zone where the tape comes into contact with the tube, to a temperature above the melting point of the material A;
means for applying local pressure to that part of the outer peripheral surface of the tube coated with the tape, in a zone located immediately downstream of the heating zone.
According to a preferred variant, the means for surface heating the tape comprise at least one infrared-type oven, preferably one operating with strips or lamps regulated in terms of power according to the temperature of the surface-heated tape. Such an oven has the advantage of being both of high performance from the energy standpoint and of being easy to regulate.
Advantageously, the winding means comprise a laying head which facilitates the process of putting the heated tape according to the invention in place. The laying head is able to rotate in a controlled manner; preferably, it comprises three mutually parallel rollers of hyperboloidal shape, the longitudinal axis of which is approximately perpendicular to the direction in which the tape runs, this itself being parallel to the axis of pivoting of the laying head. Such a configuration for the laying head considerably improves the precision and reproducibility with which the tape is put into place on the tube.
According to an additional characteristic, the means for heating the tube coated with the tape comprise a hot-air-blowing nozzle of approximately oblong cross section.
According to another characteristic, the means for applying the localized pressure comprise at least one rotating roller pressurized by an actuator.
When all the steps according to the invention are carried out continuously, using wound packages of continuous comingled yarns obtained especially by a direct process, the apparatus may furthermore comprise:
means for leading in and assembling continuous comingled yarns consisting of intimately mixed glass filaments and filaments of material A in the form of at least one sheet of parallel yarns;
means for heating the said sheet to a temperature between the melting point and the degradation temperature of the material A;
a device for impregnating the heated sheet so as to obtain a densified and laminated tape of flattened shape;
means for maintaining the tape at a temperature between the melting point and the degradation temperature of the material A as far as the means for winding the tape.
According to this variant, the winding and assembling means comprise a creel, from which packages of intimately mixed continuous comingled yarns consisting of glass filaments and filaments of material A are unwound, and at least one roller for guiding the comingled yarns.